If coaches use position players to pitch and they can't warm-up before relieving, performances will obviously suffer. Sometimes, when we have the foresight to warm-up a position player he has to bat, so either he does not get enough time to get loose or he gets cold by being on base. Very few high school teams have enough depth to have a relief staff, so this is a recurring problem. The only solution is to gain enough baseball instinct and knowledge of the pitchers on your team to know when the starters are losing their stuff and how fast each reliever can get loose.
For position players, you need to tell them to play hard catch in the field before an inning starts to get them prepared for relief.